A comparative study of frictional response of shed snakeskin and human skin

Abstract

Skin in biological systems, including humans, perform several synchronized tasks (mechanical, protective, tactile, sensory, etc.). Tribological function is among skin tasks and may determine the survivability of many species. Cross comparison of tribological functional traits of skin of different species, albeit interesting, is rarely encountered, if at all exists, in tribology literature. One interesting example is that of snake and human skins. This skin pair was the subject of many studies for transdermal drug delivery. Results in that context concluded that snakeskin is highly compatible with human skin despite apparent differences in surface structure and topology. The reported compatibility raises curious question of whether there exists frictional or tribological compatibility between the two skins and if so, under what conditions, and which context. In this work, we report, for the first time in open literature, results of a comprehensive comparative investigation of shed snakeskin and human skin with respect to tribological behaviour. To this end, we compared the frictional response of shed skin obtained from P. regius and human skin from different anatomical sites, gender, and age. The results imply that, in essence, the mechanisms governing the friction response of human skin are common to snake skin despite difference in chemical composition and apparent surface structure. In particular, both skin types display sensitivity to hysteresis and adhesive dissipation. Human skin, however, being more sensitive to hysteresis than snakeskin. One interesting finding of the study is that the ratio of the coefficients of friction for snake and human skin, when sliding on the same interface, depends on the reciprocal of their respective moduli of elasticity.

Changes in the three-dimensional microscale topography of human skin with aging impact its mechanical and tribological behavior

“…Previous experimental studies have shown that the interaction of the human skin with different materials and surfaces can show coefficient of friction over a wide range, between 0.059 and 3.7 [ 44 , 107 , 108 ]. Although our analysis does not represent the interaction between the human skin and particular materials, values of μ = 0.25 have been reported for the interaction between calf skin and nitrocellulose [ 29 ], μ = 0.22 for the contact between forearm skin and stainless steel [ 109 ], μ = 0.22 between forearm and polypropylene and between different body zones and Teflon [ 27 ], and μ = 0.19 between forearm skin and steel [ 44 ] among other similar values to the local coefficient of friction considered here. Previous work considering rigid indenters and soft substrate showed that the reaction forces on the indenter are not just due to the local friction but that an asymmetric deformation field on the length scale of the indenter can contribute a significant deformation component [ 33 , 34 , 86 ].…”

“…It operates physiologically in the large deformation regime and shows a highly nonlinear stress-strain response. The frictional behavior of human skin at the tissue scale (in the order of cm) is affected by several variables including age [25,26], anatomical region [27][28][29], contact material [30], type of contact [5,31], environmental conditions [6,26,28], and hydration of the tissue [16,32]. However, to discriminate between the relative contributions of these factors, it is advantageous to zoom in to the mesoscopic scale, in the order of hundreds of μm to a few mm.…”

Changes in the three-dimensional microscale topography of human skin with aging impact its mechanical and tribological behavior

“…Previous experimental studies have shown that the interaction of the human skin with different materials and surfaces can show coefficient of friction over a wide range, between 0.059 and 3.7 [101,43,102]. Although our analysis does not represent the interaction between the human skin and particular materials, values of =0.25 have been reported for the interaction between calf skin and nitrocellulose [29], =0.22 for the contact between forearm skin and stainless steel [103], =0.22 between forearm and polypropylene and between different body zones and Teflon [27], and =0.19 between forearm skin and steel [43] among other similar values to the local coefficient of friction considered here. Previous work considering rigid indenters and soft substrate showed that the reaction forces on the indenter are not just due to the local friction but that an asymmetric deformation field on the length scale of the indenter can contribute a significant deformation component [33,34,85].…”

“…It operates physiologically in the large deformation regime and shows a highly nonlinear stress-strain response. The frictional behavior of human skin at the tissue scale (in the order of cm) is affected by several variables including age [25,26], anatomical region [27,28,29], contact material [30], type of contact [5,31], environmental conditions [6,26,28], and wetness in the tissue [32,16]. However, to discriminate between the relative contributions of these factors, it is advantageous to zoom in to the mesoscopic scale, in the order of hundreds of m to a few mm.…”

Snake‐Inspired, Nano‐Stepped Surface with Tunable Frictional Anisotropy Made from a Shape‐Memory Polymer for Unidirectional Transport of Microparticles

“…[ 5 ] These natural prototypes inspired the development of artificial surfaces with unidirectional wetting properties allowing the transport of liquid droplets. [ 4,6,7 ] Another interesting case is the frictional anisotropy of snake scales [ 8–13 ] enabling snakes to locomote efficiently, even on slippery or inclined surfaces. While some snakes use the edges of their ventral scales to climb even trees, [ 10 ] many snakes feature oriented micron‐sized fibrils with nanoscale steps causing anisotropic friction along their body ( Figure a).…”